It is often necessary to treat a fluid with optical spectral energy which may be infrared, ultraviolet or visible light. The irradiation may be used to cause chemical, physical or biological changes, such as by inducing the reaction of two constituent chemicals, by heating or by sterilizing, respectively. The absorption of light by the fluid attenuates the light so that fluid beneath the surface is irradiated with light which is less intense than the light at the fluid surface.
U.S. Pat. No.4,831,268 to Fisch et al., U.S. Pat. No. 4,327,276 to Injushin et al., U.S. Pat. No. 1,091,221 to Henri et al. and U.S. Pat. No. 1,038,631 to Neveu all disclose methods or apparatus for treating a fluid by irradiation. None of these references disclose a method or apparatus in which the fluid is constrained to flow in a layer which is thin with respect to the direction of the irradiating rays, and thus a portion of the fluid which flows or is stagnant further from the source of light receives only that light which has been attenuated by the fluid closer to the source. The gradient of light intensity depends on the power of the source, and results in either only the surface of the fluid being treated, or an unnecessarily high intensity of light at the surface which results in an inefficient utilization of light or an adverse affect on the fluid, such as its decomposition.
U.S. Pat. No. 2,065,054 to Creighton et al., U.S. Pat. No. 1,307,500 to Keyes et al. and, U.S. Pat. No. 1,145,140 to Henri et al. all disclose apparatus for treating fluids with light in which the fluid is presented as a thin film to the irradiating light. These references disclose apparatus for radiating the light from a source in an isotropic, non-focused way. Such light which is not focused typically is not intense enough to penetrate fluids which absorb light, unless impractically high power is used.